Even the Library of Congress has created a Flickr Commons page, and the FAQ for why.
Blog post on how some teachers are using Flickr

Commoncraft Why to use Flickr

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Liz Davis Screencast

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Alan Levine/Cogdog

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Alan's Original Flickr page with more links

Fun Flickr addons

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FlickrCC Flickr pictures under the Creative Commons license
Flickr Montage creator
Retrievr draw a shape and it tries to match color and form
100 Flickr tools